The present invention relates to a glass material, in particular, a sealing glass, and a magnetic head made by bonding with the sealing glass and a magnetic recording and reproducing apparatus using the magnetic head. Moreover, the glass material of the present invention can have a high strength without increasing a melting point. Therefore, the glass material can be also used for heat resistant parts such as a pannel glass for a cathode-ray tube and a Pyrex glass, and additionally, can be further used for a pannel glass for a liquid crystal display.
Hitherto, a sealing glass which can be handled at a temperature lower than the resisting temperature of various parts has been used for bonding of core materials of magnetic heads to be mounted on magnetic recording and reproducing apparatuses such as VTR or bonding of semiconductor sensors, covering of electronic circuit parts such as IC and LSI, and sealing of electron tubes. When the resisting temperature of the parts is high, sealing glasses of SiO.sub.2 --B.sub.2 O.sub.3 type or PbO--SiO.sub.2 type are used which are relatively high in chemical endurance or mechanical strength, but when the resisting temperature is low, sealing glasses of PbO--B.sub.2 O.sub.3 type are mainly used.
At present, in the case of magnetic heads for VTR, single crystal Mn--Zn ferrites having a saturated magnetic flux density (Bs) of about 5000 gausses are used as the magnetic core materials. Since the resisting temperature of these ferrites is about 800.degree. C., glasses of PbO--SiO.sub.2 --R.sub.2 O type or ZnO--B.sub.2 O.sub.3 --SiO.sub.2 --R.sub.2 O--RO type which can perform the bonding at 700-800.degree. C. are used as the bonding glass. Here, R.sub.2 O means an alkali metal oxide and RO means an alkaline earth metal oxide. With recent development in higher performance magnetic recording and reproducing apparatuses and high-recording density magnetic recording media, Co-based amorphous metal magnetic films or sendust alloy films having a Bs of about 10000 gausses and magnetic films mainly composed of iron element and having a Bs of 12000 gausses or more, such as Fe--N or Fe--C magnetic films, have been developed for magnetic heads.
These magnetic films have a high saturated magnetic flux density while they are considerably lower in resisting temperature than Mn-Zn ferrites. Therefore, the bonding temperature is lower than 480.degree. C. for Co-based amorphous metal magnetic films, about 600.degree. C. for sendust alloy films, 500-550.degree. C. for Fe--N magnetic films, and 550-600.degree. C. for Fe--C magnetic films. Thus, a PbO--B.sub.2 O.sub.3 sealing glass which can perform the bonding at lower than these resisting temperatures is used as disclosed in JP-A-63-170240, JP-A-63-298807, JP-A-3-265539, JP-A-2-184541, JP-A-2-258649, etc. Moreover, it is proposed to use a V.sub.2 O.sub.5 --P.sub.2 O.sub.5 sealing glass as disclosed in JP-A-4-132634.
In the above conventional techniques, no sufficient consideration has been taken to satisfy simultaneously the three conditions of bonding ability at low temperatures of lower than 600.degree. C., sufficient mechanical strength and sufficient deaeration. Recently, the demand for high density recording of magnetic recording and reproducing apparatuses is further increased and the magnetic heads mounted thereon are required to have a high output and a sufficient strength to stand the use under further severer conditions. As magnetic core materials which can meet the demands, Fe--C or Fe--N magnetic films very high in saturated magnetic flux density are studied, and, hence, glasses which can carry out sealing at low temperatures of lower than 600.degree. C. must be used.
Furthermore, in order to improve tape touch between the magnetic head and the magnetic recording medium, the sliding width between the magnetic head and the magnetic recording medium must be reduced. Moreover, in order to increase recording capacity, the relative speed of the magnetic head and the magnetic recording medium must be markedly increased than usual.
For these reasons, the glass bonding part of magnetic heads is exposed to the severer use atmosphere. Therefore, the conventional sealing glass capable of performing the bonding at low temperatures are insufficient in mechanical strength and breakage sometimes occurs from the glass bonding part during the sliding of tape. That is, the glasses disclosed in JP-A-63-170240, JP-A-63-298807 and JP-A-3-265539 are insufficient in mechanical strength and can hardly stand the above-mentioned use atmosphere.
The sealing glasses disclosed in JP-A-2-184541 and JP-A-2-258649 are superior in mechanical strength, but since they contain many crystallizing components, viscosity of the glass can be adjusted with difficulty and deaeration can be controlled with difficulty. The sealing glass disclosed in JP-A-4-132634 contains P.sub.2 O.sub.5 and there is the possibility of generation of bubbles produced by water contained in the starting materials.
JP-A-1-138150 discloses a low melting point glass mainly composed of PbO, TeO.sub.2, P.sub.2 O.sub.5 and the like, and containing fluorine and rare earth metal oxides, i.e., Y.sub.2 O.sub.3, La.sub.2 O.sub.3 and Gd.sub.2 O.sub.3. However, it is hard to enhance the strength of this type of a glass because the rare earth metal oxides are incorporated into the glass structure and crystalline particles are not generated owing to P.sub.2 O.sub.5 and fluorine contained therein.